1. Field of the Invention
The present invention relates to the manufacture of a quartz glass crucible for use in the manufacture of single crystal silicon. More particularly, the present invention pertains to a structure and a method for manufacturing a quartz glass crucible having inner and outer layers of different properties.
2. Description of Prior Art
In a so-called Czochralski process for pulling a single crystal silicon, use is made of a quartz glass crucible for storing molten silicon. Such type of crucible is usually made of natural crystalline or non-crystalline quartz. The quartz is crushed and refined to produce refined quartz powder which is formed into a shape of a crucible in a rotatable mold and heated at the inner wall by means of an arc discharge while the mold is being rotated. The preformed quartz powder is molten under the heat of the arc discharge and after a predetermined time of the heating it is cooled down to form a quartz glass crucible having a smooth inner wall surface.
The crucible thus formed generally contains a lot of fine bubbles or voids in the wall so that it has an opaque appearance. It is believed that the wall structure having a lot of fine bubbles or voids shows a relatively uniform thermal conductivity. It is also believed important for making stable the process of pulling a single crystal silicon that the quartz glass crucible for use in this process has the bubble structure and a smooth inner wall surface.
The inventors have made an extensive investigation to provide a quartz crucible of improved property and already proposed a crucible of a two layer structure in U.S. Pat. Nos. 4,935,046 and 4,956,208. According to the crucible structure proposed by the U.S. patents, the crucible has an inner layer of a predetermined thickness such as 0.5 mm to 2 mm, which is substantially free from bubbles or voids and an outer layer of the aforementioned opaque structure having a lot of bubbles. The U.S. patents also disclose a novel process for manufacturing a crucible of the aforementioned two layers structure.
The quartz glass crucible disclosed by the U.S. patents has been found advantageous in that it can significantly decrease roughening of the inner wall surface of the crucible which may otherwise be produced in the process of pulling the single crystal silicon. It has also been found that the crucible structure of the U.S. patents is effective to decrease crystobalites which may be produced on the crucible in a process which uses a conventional crucible. As a result, the crucible in accordance with the U.S. patents is effective to make the process for pulling the single crystal silicon stable and to improve the production yield to a remarkable extent.
In recent years, production of super LSI has been increased. For the production of the super LSI, it is necessary to use a silicon wafer of a high quality. In order to provide a steady supply of high quality silicon wafers, it is required to provide a quartz glass crucible of high purity. In view of the requirement, proposals have been made to use powders of synthetic quartz instead of the powders of natural quartz. For example, the U.S. Pat. No. 4,528,163 discloses a quartz crucible having an outer structure made of powders of natural quartz and an inner lining layer made of powders of synthetic quartz, the inner lining layer being formed on the inner surface with a thin layer of smooth non-crystalline structure.
It should however be noted that the innermost layer of the structure disclosed by this U.S. patent is very thin and may not be thicker than 0.1 mm. The remainder of the structure has a lot of bubbles or voids. Therefore, the crucible cannot be used for a prolonged time in a plurality of processes of pulling the single crystal silicon.
The Japanese patent publication Sho 62-36974 discloses process of manufacturing a glass product from a synthetic quartz by forming silicon tetrachloride of high purity into a configuration of the product, sintering the thus formed product, heating it at the surface to have the wall of the product molten and cooling the product to obtain a glass product such as a quartz crucible. The Japanese laid-open patent application Sho 61-44793 discloses a process for manufacturing a quartz glass crucible having an inner layer made of powders of a synthetic quartz. According to the teaching of the Japanese patent application, a molded crucible is prepared from material powders to have an inner layer made of powders of a synthetic quartz containing more than 200 ppm of OH group and an outer layer made of powders of natural crystalline quartz containing less than 100 ppm of OH group, and the molded crucible is then heated to be molten while it is rotated.
It should however be noted that the process disclosed by either of these Japanese patent applications cannot provide a crucible having a wall of satisfactory transparency even if the inner wall surface of the crucible is molten as described so that the crucible produced by this process cannot provide a stable process for pulling a single crystal silicon. The crucible produced by the process taught by either of the aforementioned two Japanese patent applications does not have a transparent inner layer of a thickness greater than 0.5 mm and substantially free from bubbles or voids as in the crucibles disclosed by the aforementioned U.S. Pat. Nos. 4,935,046 and 4,956,208.
In forming a silica glass layer from a synthetic material, use may be made of crystalline synthetic silica as a starting material. However, this process may be uneconomical because the crystalline silica is very expensive since it is made by a process including a lot of steps. For example, it is usually made of ester silane or sodium silicate through hydrolysis, or of silane halides through hydrolysis to produce non-crystalline silica which is thereafter crystallized by means of a high temperature devitrification using a crystallizing core such as an alkali.
Synthetic silica powders are generally of non-crystalline structure so that they can be economically used for forming a glass layer. However, it has been recognized that powders of non-crystalline silica do not have a uniform melting point so that it is difficult to obtain a smooth surface in forming a glass layer.